An example of a prior art system is shown in FIG. 1. P0 is assumed to be the preset starting location or the last-calculated on-road location of the vehicle. Taking P0 as a reference, the travel distance and the direction change are measured by accumulation or integration of the movement of the vehicle, and the results of the measurement are used for the determination of the accumulation type data of present location P1. The accumulation type location data P1 is then compared with the road map data that is stored in advance, and on-road data r1 to r3 which are within a predetermined error range E are determined, and the accumulation type location data P1 is correlated with road data r.sub.1 to r.sub.3 to find on-road locations P11 to P13 which are within the predetermined error range E, and an evaluation function is determined for each of the on-road locations P11 to P13, and the on-road location P12 having the highest evaluation function is selected and displayed on the display means. The on-road locations P11 to P13 are used as references for the determination of the accumulation type location data and the on-road locations in the next cycle.
In FIG. 1, F shows the locus of the accumulated data location, and G shows the actual route of the vehicle.
A problem associated with the prior art is that a plurality of on-road locations may be generated (this is the case if there are a plurality of road data within the predetermined error range from the accumulated data location), and all of them are stored as the present locations of the vehicle. The number of present locations that are stored in the memory thus increases with time. Moreover, the accumulated data location determined from the travel distance and the direction change contain considerable errors, and the on-road locations determined from the accumulated data locations therefore are not accurate.